Wavelength Division Multiplexing (WDM) is currently in widespread use, in which multiple optical signals having different wavelengths are transmitted through one transmission line to increase the transmission capacity. In the WDM, the optical output power after the multiplexing of the optical signals is increased to a high level, for example, to about +20 dBm even if the optical output power per one wavelength is low, for example, is equal to a few dBm. Accordingly, it is necessary to take measures against the high optical output power.
For example, optical amplifier apparatuses used in optical communication systems include dispersion compensation devices that compensate dispersion characteristics occurring in the transmission lines. The optical amplifier apparatuses perform, for example, detection of connection failure in the dispersion compensation devices as the above measures.
FIG. 15 illustrates an example of the configuration of an optical communication system in related art. Referring to FIG. 15, the optical communication system includes optical amplifier apparatuses 10 and 20. The optical amplifier apparatus 10 is connected to the optical amplifier apparatus 20 via a transmission line 1. An optical signal amplified by the optical amplifier apparatus 10 is provided to the optical amplifier apparatus 20 through the transmission line 1. Since the optical amplifier apparatus 10 includes the same configuration as that of the optical amplifier apparatus 20, the configuration of the optical amplifier apparatus 20 will now be described.
The optical amplifier apparatus 20 includes couplers (CPLs) 21 and 22, a variable optical attenuator (VOA) 23, photodiodes (PDs) 24 and 25, a dispersion compensation device 26, and an optical amplifier module 27. Each of the CPLs 21 and 22 is a device that splits an optical signal.
The CPL 21 splits an optical signal provided from the optical amplifier apparatus 10 into signal components to provide one signal component to the PD 24 and provide the other signal component to the VOA 23. The CPL 22 splits an optical signal provided from the VOA 23 into signal components to provide one signal component to the PD 25 and provide the other signal component to the dispersion compensation device 26.
The VOA 23 is a device that attenuates an optical signal. The VOA 23 receives an optical signal from the CPL 21, attenuates the received optical signal, and provides the attenuated optical signal to the CPL 22.
The PD 24 is a device that receives an optical signal from the CPL 21 to convert the received optical signal into an electrical signal. The PD 24 provides the electrical signal to a monitor. The PD 25 is a device that receives an optical signal from the CPL 22 to convert the received optical signal into an electrical signal. The PD 25 provides the electrical signal to the monitor.
The dispersion compensation device 26 causes an appropriate delay for every wavelength in an optical signal that is received from the CPL 22 to compensate the dispersion existing in the optical signal. The dispersion compensation device 26 is connected to a To_DCF port and a From_DCF port.
The dispersion compensation device 26 receives an optical signal output from the CPL 22 through the To_DCF port to perform dispersion compensation to the received optical signal. The dispersion compensation device 26 provides the optical signal subjected to the dispersion compensation to the optical amplifier module 27 through the From_DCF port.
In order to simplify the optical communication system, to simplify the operation, and to decrease the number of menus in the dispersion compensation device, it is effective to use a dispersion compensation device including a directional device, such as a circulator as the dispersion compensation device 26. The dispersion compensation device including a directional device is, for example, a variable dispersion compensation device.
The optical amplifier module 27 is a device that receives an optical signal from the dispersion compensation device 26, amplifies the received optical signal, and outputs the amplified optical signal. The optical amplifier module 27 includes a PD. The optical amplifier module 27 converts the optical signal into an electrical signal with the PD and provides the resulting electrical signal to a monitor.
In detection of any fault in the dispersion compensation device 26, the optical amplifier apparatus 20 detects the difference between the electrical signal output from the PD 25 and the electrical signal output from the PD in the optical amplifier module 27. The optical amplifier apparatus 20 compares the detected difference with a given value to determine whether any connection failure occurs in the dispersion compensation device 26.
For example, if the optical amplifier apparatus 20 detects a connection failure, the optical amplifier apparatus 20 forcedly shuts down the optical amplifier module 27 to prevent an abnormal optical signal from being output from the optical amplifier module 27.
However, the above technology in the related art has a problem in that the optical amplifier apparatus cannot accurately detect a failure concerning the dispersion compensation device when the dispersion compensation device including a directional device is used. FIGS. 16 and 17 illustrate problems in the related art.
A problem in the related art will now be described with reference to FIG. 16. The dispersion compensation device 26 in FIG. 16 includes a circulator 26a and a device 26b performing the dispersion compensation. An optical signal received through a port (1) of the dispersion compensation device 26 is provided to the device 26b through the circulator 26a. An optical signal output from the device 26b is output from a port (2) through the circulator 26a. 
With the fault detection method in the related art, since “the value of the monitor for the PD 25 is normal” and “the value of the monitor for the optical amplifier module 27 is abnormal” in a case in which a failure occurs in the device 26b and a case in which a failure, such as port disconnection, occurs in the To_DCF port or the From_DCF port, it is not possible to discriminate the above two cases.
Another problem in the related art will now be described with reference to FIG. 17. In the dispersion compensation device 26 illustrated in FIG. 17, the To_DCF port and the From_DCF port are connected in a manner opposite to that in the connection in FIG. 16 because of the circulator. In the connection in FIG. 17, an optical signal received through the port (2) is output from the port (1) not through the device 26b. 
When the To_DCF port and the From_DCF port are connected in the opposite manner, it should be determined that a failure occurs because the optical signal does not pass through the device 26b and expected dispersion characteristics are not received. However, since the amount of optical loss is small when the optical signal does not pass through the device 26b, as in the case in FIG. 17, it is not possible to determine that the abnormal state occurs only from the value of the monitor for the PD 25 and the value of the monitor for the optical amplifier module 27 and, thus, the abnormal state cannot be detected.